1. Field of the Invention
This invention relates to an electrical filter, and more particularly to an electromechanical filter provided with a mechanical resonator.
2. Description of the Prior Art
An electromechanical filter is well-known in the field of telecommunication filter and is now used widely in a radio receiver, a carrier telephone equipment, etc. The electromechanical filter is, in principle, made up of an excitation part mainly composed of a mechanical exciter, a resonance part mainly composed of a mechanical resonator and a coupling part for mechanically coupling together the excitation and the resonance part. Especially in an electromechanical filter of 200 to 500 KHz for use as a channel filter is an aeronautical or maritime radio communication receiver, carrier telephone equipment or the like, use is made of a columnar metal rod resonator which utilizes a longitudinal or torsional vibration mode as a dominant mode. The excitation part is generally comprised of a transducer and an exciter; and a typical example of this excitation part is what is called the Langevin structure that an exciter composed of a columnar metal rod is attached to either side of a disc-shaped transducer using an adhesive binder, such as epoxy resin or the like, as set forth in "The Electro-Acoustic Sensitivity of Cylindrical Ceramic Tubes", The Journal of the Acoustic Society of America, No. 3, pp 421, May 1954. Another example of the existing excitation part is a known flexural structure in which a strip-shaped transducer, for example, a peizoelectric ceramic, is attached to one side of an exciter formed with a strip of metal, using epoxy resin or like adhesive binder; however, this flexural structure is susceptible to a bending vibration mode other than the longitudinal vibration mode utilized as the dominant mode and the resulting spurious response extremely degrades the filtering characteristic.
With recent marked enhancement of the characteristics of transducers, including a lead-zirconate-titanate, a transducer of excellent temperature stability is now available. In either of the conventional Langevin type and the flexural structure, however, an organic material of poor temperature stability is employed as an adhesive binder for coupling of the transducer and the exciter, so that the temperature stability of the filter is still low as a whole.
Further, in order to achieve mechanical impedance matching between the mechanical resonator and the exciter, the mass of the latter must be made half that of the former; but, since the conventional excitation part is composed mainly of a metallic exciter of a large density, the excitation part becomes too small, especially in the high-frequency use, introducing difficulty in its fabrication.